Synthesis And Electrochemical Properties Of The Li_4-2xMg_xB₇O₁₂Cl Solid Electrolytes

Lithium-ion batteries have advantages such as long cycle life,high energy density,high working voltage,and environmental friendliness.At present,the electrolyte materials used in commercial lithium-ion batteries are mainly organic solvents such as dimethyl carbonate（DMC）and ethylene carbonate（EC）,which are prone to leakage,volatilization,combustion,explosion and other safety concerns.The utilization of noncombustible or nonflammable solid electrolytes in all solid lithium-ion batteries can in theory solve the safety issues of lithium-ion batteries,and the key lies in the properties of solid electrolyte materials.Solid electrolytes usually require high ion conductivity and high stability.For halides,the ion conductivity is relatively high,but they are generally unstable in air.For oxides,the stability is good,but the ionic conductivity is a bit low.Considering the high electronegativity of oxygen element,the bond between oxygen and promising metal cation is strong,which leads to high stability.For the bond between lithium and halogen elements,the interaction is weak,which is beneficial for the lithium-ion transport.Therefore,oxyhalides are expected to be a choice for solid electrolyte materials with high conductivity and stability.This project focuses on the doping modification and electrochemical performance of Li₄B₇O₁₂Cl.The details are introduced as below:（1）Doped Ion Screening and Solid-state Synthesis of Li₄B₇O₁₂Cl Solid Electrolytes:According to the basic consideration of similar ionic radius and similar ion coordination environment,we tried to dope Sb and Te ions at the B position and Mg ions at the Li position,and screened the doped ions of Li₄B₇O₁₂Cl solid electrolyte according to the measurement results of X-ray diffraction,and Mg ions were selected among the doping trials of Sb ions,Te ions,and Mg ions.The Li_4-2xMg_xB₇O₁₂Cl solid electrolyte compounds were successfully prepared at 800^oC.The phase purification of the as-synthesized Li_4-2xMg_xB₇O₁₂Cl was confirmed by powder X-ray diffraction.The influence of the doping ratios of Mg ions on the crystal cell parameters of Li₄B₇O₁₂Cl was analyzed.In addition,Li_3.6Mg_0.2B₇O₁₂Cl samples were selected for SEM analysis of the micro morphology of the sample surface.The experiment shows that as the doping ratio increases,the main XRD peaks corresponding to Li_4-2xMg_xB₇O₁₂Cl shifts towards higher angles,indicating the values of the crystal cell parameters gradually decrease with higher x.This confirms the successful introduction of Mg element into the Li site.The powder sample is mainly composed of aggregated spherical particles and some rod-shaped particles,with a particle size ranging from 1 to 5μm.There are small gaps between the particles,and the sample exhibits good macroscopic uniformity on the surface.（2）Electrochemical Properties of Li_4-2xMg_xB₇O₁₂Cl Solid Electrolytes:The electrochemical performances of the as-obtained Li_4-2xMg_xB₇O₁₂Cl solid electrolyte materials were studied.The generation of Li vacancies allows ions to transition,providing more transport pathways for ions.The ion conductivity of undoped sample Li₄B₇O₁₂Cl is 3.03×10^-7S cm^-1at room temperature.At a doping ratio of x=0.2,Li_3.6Mg_0.2B₇O₁₂Cl achieved the highest ion conductivity of 2.81×10^-5 S cm^-1at room temperature.The ion conductivity of Li₄B₇O₁₂Cl at room temperature was increased by two orders of magnitude.The Li_3.6Mg_0.2B₇O₁₂Cl sample reaches a higher ion conductivity of 2.37×10^-4 S cm^-1 at 50?C,an activation energy of 0.63 e V,and an electrochemical stability window reaching nearly 4 V.The average lithium-ion migration numbers of undoped samples Li₄B₇O₁₂Cl and Li_3.6Mg_0.2B₇O₁₂Cl electrolytes at room temperature are 0.61 and 0.72,respectively.The symmetrical batteries assembled with Li_3.6Mg_0.2B₇O₁₂Cl could maintain stable voltage input and output even after 60 cycles both at room temperature and 50?C.The experiment confirmed that Mg ion doping improved the electrochemical performance of Li₄B₇O₁₂Cl solid electrolyte.（3）Electrochemical Properties of batteries assembled with Li3.6Mg0.2B7O12Cl solid electrolyte:Based on the research results of the electrochemical performance of Li_4-2xMg_xB₇O₁₂Cl solid electrolyte material,a symmetrical battery was further assembled using Li_3.6Mg_0.2B₇O₁₂Cl.At room temperature and 50?C,the hysteresis voltages of the battery are 0.3m V and 0.02 m V,respectively,with lower voltage hysteresis and almost 99.7%and 99.8%Coulombic efficiency,respectively.Both have good retention rates and exhibit good cycling performance.This indicates that the electrochemical performance of Li₄B₇O₁₂Cl solid electrolyte has been improved by doping Mg ion,and the prepared Li_4-2xMg_xB₇O₁₂Cl can be used as a high stability and high ion conductivity solid electrolyte material for all solid lithium-ion batteries.